Quadruphonic disc recording system utilizing single sideband modulation

ABSTRACT

As described hererin, sound information comprising four discrete audio signals is recorded on a single groove, two-channel record disc. To this end, two of the four signals are added to produce a sum signal and subtracted to produce a difference signal. The difference signal is employed to amplitude modulate a carrier signal and the upper sideband thereof is added to the sum signal and applied to one input terminal of a recording transducer. The transducer modulates one channel of the record groove in response to the sum signal and the upper sideband signal applied thereto. A similar operation is performed upon the remaining two signals, and the transduced modulates the second channel of the record groove in response to the sum signal and the upper sideband applied thereto.

United States Patent 1 1 1111 3,869,583 Bauer et al. 1 Mar. 4, 1975 154]QUADRUPHONIC DISC RECORDING 3,067,292 12/1962 Minter 179/1004 ST SYSTEMUTILIZING SINGLE SIDEBAND 4/1963 Logan, Jr. et a1 325/50 4 1966 Richman325/50 MODULATION 3.401.237 9/1968 Takayanagi 179/1004 ST [75]Inventors; Benjamin B, Bauer, Stamford; 3,686,471 8/1972 Takahashi179/1004 ST Bar iiel grat ereallilx, X/Cilton; Emil OTHER PUBLICATIONSone amen a O News for the Aniophile, Burstein, Radio-Electronics. [73]Assignee: Columbia Broadcasting System, Inc., pages 38-39, April, 1958.

' New York, NY.

Primarv E.\'aminerRaymond F. Cardillo Jr. F D 19 [22] ec 73 Attorney,Agent, or Firm-Spencer E. Olson [21] Appl. No: 428,294

Related US. Application Data [57] ABSTRACT [63] Continuation of Ser. No169219 Aug 5 971 As descrlbed hererin, sound information comprisingdbflndunc four discrete audio signals is recorded on a single 1 groove,two-channel record disc To this end, two of 52} us. c1 179 1004 ST, thefour Signals are added ProdIce a Sum Signal and 179/] GQ |79/100 1 TD179/1004 M subtracted to produce a difference signal. The differ- I l][m Cl Gllb 3/74 ence signal is employed to amplitude modulate a cari IFie'ld 100 1 TD rier signal and the upper sideband thereof is added to il79/H RP 15 5 FE the sum signal and applied to one input terminal of a 36 137 1 recording transducer. The transducer modulates one channel ofthe record groove in response to the sum [S6] References Cited signaland the upper sideband signal applied thereto. A similar operation isperformed upon the remaining UNITED STATES PATENTS two signals, and thetransduced modulates the second 1.777.900 1/1957 Couvan 325/136 channelof the record groove in response [0 the um g tg$ signal and the uppersideband applied thereto. 211.1 ue t g 2 Claims, 3-Drawing Figures 24 1zuy E L9 P t=. P R qc E s s 0 i SINGLE swswm MoouLAroR l2 0- (LFLb) (224* b PHASE f EQUALIZER eu u gt r lps MATRIX '8 9 I9 iL +L l Lb MOD.

F I6 CARRIER 32" FULL WAVE AMPLITUDE ig- 5 nzrecroal 1'" MODULATOR HIGHPASS FILTER I (Rf +'R t 19 +1400.

' CARRIER MATRIX PHASE R EOUALIZINB b CIRCUIT PATENTED 4|975 sum a 95 2Lf Lb (CARRIER) (Lf-l-Lb)+ MOD. CARRIER' PREAMPLIFIER PREAMPLIFIER (R-0-R )+MOD. CARRIER FREQUENCY (H3) FIG. 2

LOW PASS FILTER HIGH PASS FILTER FILTER PHASE DE-EMPHASIS 62 EQUALIZINGCIRCUIT 66 cmcuw I l MATRIX I l I AM DE-EMPHASIS 8 DETECTOR CIRCUITPHASE DE-EMPHAS|S EQUALIZING CIRCUIT 62' CIRCUIT I 1 1 I EM 61 60 MATRIXI I Rb 6 7? AM DE-EMPHASIS 58' DETECTOR cmcun INVENTORS BENJAMIN IB.BAUER,

their DANIEL W. GRAVEREAUX 8| EMIL L. TO

ATTORNEYS 1 QUADRUPHONIC DISC RECORDING SYSTEM UTILIZING SINGLE SIDEBANDMODULATION This is a continuation, of application Ser. No. 169,219,filed August 5, 1971, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to stereophonic discrecording systems and, more particularly, to a new and improvedrecording system adapted to record four independent channels of audioinformation on a two-channel disc record.

In systems for recording stereophonic information comprising two signalsin a single groove record disc, it is customary to modulate the twochannels (left and right) of the record groove in two perpendiculardirections. Thus, the groove variations in one channel represent onefunction of the two signals and those in the other channel representanother function of the two signals. Typically, the groove is cut withmodulation in each channel of the groove representing each signal andwith lateral modulation representing the sum of the signals and verticalmodulation representing the difference between the signals.

Since the commercialization of stereophonic phonograph soundreproduction in about 1958, efforts have been made to record three ormore signals on a twochannel record for the obvious reason that thesound distribution of the reproduced information will improve as thenumber of signal sources increases. For example, three channel discrecording systems have been proposed in which a selected fraction of athird signal corresponding to the sound detected by a centrally disposedmicrophone is added to the two signals corresponding to the sounddetected by spaced left and right microphones to record a dependent sum,or center, channel on the stereophonic record. Alternatively, it hasbeen proposed to subtract a selected fraction of the third signal fromeach of the other two signals and to record a dependent differencechannel on the stereophonic record. However, such a difference channelis not compatible with monophonic players because the vertical groovemodulation cancels for monophonic reproduction, nor with stereophonicplayers which produce an unnatural sound during reproduction.

SUMMARY OF THE INVENTION It is a principal object of the presentinvention to provide a method and apparatus for recording four discretechannels of audio information on a two-channel disc record, and forreproducing the four discrete channels upon playback of the record.

Another object of the invention is to provide a system for recordingfour discrete channels of audio information on a two-channel disc recordwhich may be reproduced on a conventional stereophonic phonographwithout loss of any sound contained in the original four signals.

These and other objects of the invention are accomplished by processingfour discrete audio signals and modulating each channel of a two-channeldisc record of the conventional 4545 modulation type in accordance withseparate functions of two ofthe signals. For convenience of description,the four discrete signals correspond to the positions in a listeningarea at which four loudspeakers would be placed to respectivelyreproduce the four signals; that is, they may be characterized as leftfront (L right front (R left back (L and right back (R In particular,selected pairs of the audio signals, for example, L; and L and R; andR,,, are added to produce sum signals and subtracted to producedifference signals. The difference signals are employed to amplitudemodulate carrier signals and the resultant carrier signals are combinedwith their respective sum signals. The combined sum signals andamplitude modulated carrier signals are then recorded on the separatechannels of a two-channel stereophonic record.

In a preferred embodiment of the invention, only the upper sidebands ofthe difference signal modulated carrier signals are combined with theirrespective sum signals and recorded on the separate channels of therecord. To compensate for the envelope distortion of the single sidebandcarrier signals, envelope processing circults are provided to process orpredistort the difference signals prior to developing the singlesideband. In addition, equalizing networks are provided to maximize themodulation in the upper frequency ranges of the difference signals.

In a preferred embodiment of a record reproducingsystem, the sum signalsare separated from the difference signal modulated carrier signals, thedifference signals are detected, and the sum and difference signals areagain added and subtracted to produce four audio signals correspondingto the original four channels for application to four appropriatelypositioned loudspeakers.

BRIEF DESCRIPTION OF THE DRAWINGS In the Drawing:

FIG. 1 is a block diagram illustrating a preferred embodiment of arecording system arranged according to the invention for recording fourdiscrete channels of audio information on a two-channel disc record;

FIG. 2 is a diagram illustrating the frequency distribution of the sumand the single sideband carrier signals recorded on one channel of atwo-channel stereophonic record; and

FIG. 3 is a block diagram illustrating a preferred em bodiment of areproducing system arranged according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In a preferred embodiment of afour-channel recording system arranged according to the presentinvention, as shown in FIG. 1, a pair of matrix circuits l0 and 10'receive four audio signals from a suitable four-channel source, such asa master tape, (not shown) via input terminals 12, 14 and 12', 14,respectively. As illustrated, the four input signals are designated leftfront (L left back (L right front (R and right back (R and eachcorresponds to the location in a listening area in which they are to bereproduced. The processing circuitry for the L and L,, signals will bedescribed in detail. As will be apparent hereinafter, the L, and Lsignals are recorded in the left channel of a sterophonic record and theR, and R signals are recorded in the right channel. The components ofthe processing circuitry for the R and R signals corresponding to thecomponents of the L, and L signal. processingcircuitry are designated byprime numbers.

Within the matrix circuit 10, which may be of conventional constructionand, accordingly, need not be .described herein, the L; and L signalsare added together to supply a sum signal (L L,,) to a channel 16and'subtracted'to supply a difference signal (L; L,,)

to a channel 18. As'described hereinafter, the sum signal (L;+L isrecorded in the left channel ofa stereophonic record and the uppersideband of a carrier signal modulated by the difference signal (L, L;,)is also recorded in the'left channel of the stereophonic record. In'likemanner, the sum signal (R;+ R' developed in the matrix l'and the uppersideband of a carrier signal modulated by the difference signal (R; Rare recorded in the right channel of the stereophonic record.

The sum signal (L; L,,) is applied via a phase equalizing circuit 19 toa buffer amplifier 20, whereas the difference signal (L; L is appliedinitially to an equalizer circuit 22. The phase equalizer circuit 19 isprovided to maintain coincidence between the sum sig nal and thedifference signal, which is phase-altered by reason of the processing towhich it is subjected, as will be apparent hereinafter.

The equalizer circuit 22 attenuates the components of the differencesignal (L; L,,) ranging in frequency from'about 20 to about 1 kHz tocompensate both for the fact that some of the lower sideband signal istransmitted to the output of the processing circuitry because of themethod employed for producing single sideband. Thesignal-to-noise ratioof the composite signal is improved by pre-emphasizing the highfrequency components of the difference signal (in which the spectralenergy is less than for mid-range frequencies) and correspondinglyde-emphasizing the signal during playback so that a flat frequencyresponse is attained and the record noise subsequently attenuated. Themethod utilized to generate single sideband is to pass a double sidebandsignal with carrier through a high-pass filter having a cutoff frequencyequal to the carrier frequency. Consequently, the audio level for lowfrequencies must be attenuated with respect to the high frequencies sothat the modulation percentage in the double sideband region is the sameas in the single sideband region for constant level input. These signalsranging in frequency from about 17 to 20 kHz will be transmitted, albeitwith attenuation. From the equalizer circuit 22 the difference signal issupplied to an envelope processor circuit 24 which includes an amplitudemodulator 26, a highpass filter 28, a full wave detector 30 and aninverter 32. The present invention utilizes a single sideband modulatorto record the difference signal (L L,,) on the same channel (left) ofthe stereophonic record groove that the sum signal (L -l- L;,) isrecorded. As is understood in the art, when a sideband is removed froman amplitude modulated carrier signal to develop a single sidebandsignal with carrier, the envelope of the single sideband signal isin theshape of a poid and thus represents a distorted replica of thedifference audio signal. In order to compensate for such distortion ofthe envelope of the single sideband signal, it is a feature of thepresent invention to predistort the difference signal (L,- L in theprocessor 24.

Within the processor 24, therefore, the difference signal (L,' L,,) isapplied to one input terminal of the amplitude modulator 26 wherein thesignal modulates a carrier signal having a frequency of 20 kHz. Thedeveloped modulated carrier signal is then applied to the highpassfilter 28 having a cutoff frequency of 20 kHz which rejects at rapidrate frequencies below 20 kHz and passes without attenuation the uppersideband of the modulated carrier signal. The transmitted sidebandsignal is then applied to the full wave detector circuit 30 whichcomprises, for example, a full wave rectifier and lowpass filter and,accordingly, detects only the envelope of the transmitted sidebandsignal. As above described, the detected signal constitutes a distortedreplica of the difference signal (L, L,,).

The inverter 32 inverts the detected and distorted difference signal andapplies the inverted signal to one input terminal of an amplitudemodulator 34, composing the first component in a single sidebandmodulator 36. In the modulator 34, the inverted and distorted differencesignal modulates a 20 kHz carrier signal applied to the other inputterminal of the modulator. A

highpass filter 38 having a cutoff frequency of 20 kHztransmits only theupper sideband of the difference signal amplitude modulated carriersignal. By virtue of the inherent distortion in the modulation andfiltering process, the envelope of the transmitted upper sideband signalis distorted with the result that the previous distortion is cancelledand the envelope of the resultant signal is an exact replica of thedifference signal (L; L,,).

From the filter 38, the upper sideband signal with carrier fs suppliedto a buffer amplifier 40 having its output terminal connected to oneinput terminal of a summing network to the other input terminal of whichthe output terminal of buffer amplifier 20 is applied. In the summingnetwork 41, the sum signal (L,+ L,,) and the (L; L single sidebandsignal with carrier are added together and supplied to one inputterminal of a recording transducer 42. In this case, the transducer 42is arranged to modulate the left channel of the record groove by meansof a stylus 43 in response to the sum signal (1 L,,) and the (L;- Lsingle sideband signal with carrier supplied thereto.

In like manner, the sum signal (R, R,,) and the upper sideband signalwith carrier having an envelope representative of the difference signal(R, R are added together in a summing network 41' and then applied tothe other input terminal of the recording transducer 42. The transducer42 is arranged to modulate the right channel of the record groove inaccordance with the sum signal (Rf+ R and the (R, R single sidebandsignal withcarrier supplied thereto.

Referring now to FIG. 2, there is shown the frequency distribution ofthe sum signals (1 L and the difference signals (L, L,,). The sum signalis recorded as a regular audio signal in the respective channel of therecord groove and the difference signal is recorded as single sidebandmodulation with carrier in the same channel of the record groove. Thetotal bandwidth extends from 30 to 35 kHz. The distribution of thefrequency components of the right channel signal is identical to thatshown for the left channel.

In a preferred embodiment of a four-channel record reproducing systemarranged according to the present invention, as shown in FIG. 3, atwo-channel disc phonograph pick-up transducer 44 includes a stylus 45that responds to the modulations in the left and right channels of arecord disc recorded in the above-described manner to supply the sumsignal and the (L, L,,) single sideband signal with carrier, and the sumsignal (R, R together with the (R; R single sideband signal withcarrier, along a pair of conductors 46 and 48, respectively, to a pairof preamplifiers 50 and 52, respectively. Like the recording system, thecomponents of the processing circuitry for the left and right channelsare identical, and accordingly, only the processing circuitry for theleft channel information willbe described herein. The correspondingcomponents in the right channel are designated by prime numbers.

The preamplifier 50 amplifies the sum signal (L, L,,) and (L; L singlesideband signal with carrier and applies the amplified signal to both alowpass filter 53 and to a highpass filter 54. The filters 53 and 54separate the sum and single sideband signals and supply the separatedsignals to a phase equalizing circuit 56 and to an AM detector circuit58, respectively. The phase equalizing circuit 56 is provided tomaintain coincidence between the sum signal and the (L] L,,) singlesideband signal with carrier which is altered in phase by virtue of theadditional circuits to which it is subjected, as will be apparenthereinbelow. From the circuit 56, the sum signal is applied by way of adeemphasis circuit 60, conventional in stereophonic record reproducingsystems, to one input terminal of a matrix 62.

In the AM detector circuit 58, which may comprise, for example, a diodenetwork and a lowpass filter, the envelope of the (L; L single sidebandsignal with carrier is detected, the detected signal being an exactreplica of the (L, -]L,,) signal produced by the matrix in the recordingprocess (FIG. 1). The difference signal (L L,,) is then applied by wayof a de-emphasis circuit 64 to the other input terminal of the matrix62. in the matrix 62, the sum signal (L;+ L,,) is combined with thedetected difference signal (L, L,,) in the conventional manner toreproduce at two pairs of output terminals 66 and 68 the left front (Lsignal and the left back signal (L,,). The right front (R signal and theright back (R signal are reproduced in like manner at the two pairs ofoutput terminals 66' and 68 of the matrix 62. Although not shown, thesesignals may then be supplied through power amplifiers to four spacedloudspeakers positioned at the left front, right front, left back andright back corners of the listening area, so that the spatial relationof the original four-channel sound is reconstructed during reproduction.

In operation, the matrices 10 and 10 receive signals L,, L,,, R, and Rand convert the signals into sum signals (L;+ L and (Rf+ R anddifference signals (L; L and R,- R respectively. The sum signal (L Ltogether with a (L; L single sideband signal with carrier, is recordedin the left channel of a stereophonic record and the sum signal (R;+ Rtogether with a (R,- R,,) single sideband carrier, is recorded in theright channel of the record. In the reproduction of the record, the sumand difference signals in each channel are separated and the differencesignal detected. Thereafter, the signals are combined in the matrices 62and 62' and reproduced as the original four audio signals, L L,,, R, andR Although the invention has been described herein with reference to aspecific embodiment, many modifications and variations therein will bereadily apparent to those skilled in the art. Accordingly, all suchvariations and modifications are included within the intended scope ofthe invention as defined by the following claims.

We claim:

1. Apparatus for recording four channel signals in a single groove cutin a record disc, said apparatus comprising:

matrix circuit means for producing first and second signalsrepresentative of the sum and the difference, respectively, of the firstand second channel signals and also third and fourth signalsrepresentative of the sum and difference, respectively, of the third andfourth channel signals,

first and second envelope processor means to which the second and fourthdifference signals produced by said matrixing circuit means arerespectively applied,

each of said envelope processor means including amplitude modulatormeans for modulating a carrier signal in accordance with the amplitudeof the applied difference signal,

filter means for transmitting the upper sideband of the amplitudemodulated signal, and

detector means for detecting the envelope of the upper sideband withcarrier signal to produce a distorted replica of the difference signal,

first and second equalizing circuit means for respectively attenuatingthe low frequency components of the second and fourth difference signalsprior to application to the amplitude modulator means of theirrespective envelope processor means to compensate for the presence oflower sideband signal information in the output of said filter means,

first and second output single sideband modulator means for modulating arespective carrier signal in accordance with the distorted replica ofthe difference signal produced by said first and by said second envelopeprocessor means, respectively, to produce single sideband with carriersignals having envelopes corresponding to their respective differencesignals, said carrier signals each having a frequency such as to causethe upper sidebands of said modulated signals to be outside thefrequency range of said first and third sum signals,

first means for combining the first sum signal with the upper sidebandof the single sideband with carrier signal produced by said first outputsingle sideband modulator means,

second means for combining the third sum signal with the upper sidebandof the single sideband with carrier signal produced by said secondoutput single sideband modulator means, and

means for simultaneously recording the output signal of said firstcombining means on one wall of a single groove of the record disc andthe output signal of said second combining meanson the other wall of thegroove.

2. A system for recording first, second, third and fourth channelsignals in a single groove cut in a record disc, said system comprising:

matrix circuit means for producing a first sum signal and a firstdifference signal representative of the sum and difference,respectively, of the first and second channel signals and also a secondsum sig nal and a second difference signal representative of the sum anddifference, respectively, of the third and fourth channel signals,

first and second single sideband modulator means respectively responsiveto said first and to said second difference signals for modulating arespective carrier signal in accordance with the amplitude of said firstand second difference signals, respectively, said carrier signals eachhaving a frequency such as to cause the upper sidebands of saidamplitude modulated signals to be outside the frequency range of saidfirst and second sum signals, i said first and second single sidebandmodulator ;means each including output single sideband modulator means,envelope processor means recarrier signal for producing a distortedreplica of the difference signal, and means for applying said distortedreplica of the difference signal to the output single sideband modulatoras a modulating signal,

first and second equalizing circuit means for attenuating the lowfrequency components of the first and second difference signals prior toapplication to their respective second amplitude modulator means andoperative to compensate for the pres ence of lower sideband signalinformation in the output of said filter means thereby to makesubstantially equal the modulation percentage in the output singlesideband modulator means for all frequency components,

first means for combining the first sum signal with the output signal ofthe first single sideband modulator means,

second means for combining the second sum signal with the output signalof the second single sideband modulator means, and

means for simultaneously recording the output signal of said firstcombining means on one wall of the single groove of the record disc andthe output signal of said second combining means on the other wall ofthe groove.

1. Apparatus for recording four channel signals in a single groove cutin a record disc, said apparatus comprising: matrix circuit means forproducing first and second signals representative of the sum and thedifference, respectively, of the first and second channel signals andalso third and fourth signals representative of the sum and difference,respectively, of the third and fourth channel signals, first and secondenvelope processor means to which the second and fourth differencesignals produced by said matrixing circuit means are respectivelyapplied, each of said envelope processor means including amplitudemodulator means for modulating a carrier signal in accordance with theamplitude of the applied difference signal, filter means fortransmitting the upper sideband of the amplitude modulated signal, anddetector means for detecting the envelope of the upper sideband withcarrier signal to produce a distorted replica of the difference signal,first and second equalizing circuit means for respectively attenuatingthe low frequency components of the second and fourth difference signalsprior to application to the amplitude modulator means of theirrespective envelope processor means to compensate for the presence oflower sideband signal information in the output of said filter means,first and second output single sideband modulator means for modulating arespective carrier signal in accordance with the distorted replica ofthe difference signal produced by said first and by said second envelopeprocessor means, respectively, to produce single sideband with carriersignals having envelopes corresponding to their respective differencesignals, said carrier signals each having a frequency such as to causethe upper sidebands of said modulated signals to be outside thefrequency range of said first and third sum signals, first means forcombining the first sum signal with the upper sideband of the singlesideband with carrier signal produced by said first output singlesideband modulator means, second means for combining the third sumsignal with the upper sideband of the single sideband with carriersignal produced by said second output single sideband modulator means,and means for simultaneously recording the output signal of said firstcombining means on one wall of a single groove of the record disc andthe output signal of said second combining means on the other wall ofthe groove.
 2. A system for recording first, second, third and fourthchannel signals in a single groove cut in a record disc, said systemcomprising: matrix circuit means for producing a first sum signal and afirst difference signal representative of the sum and difference,respectively, of the first and second channel signals and also a secondsum signal and a second difference signal representative of the sum anddifference, respectively, of the third and fourth channel signals, firstand second single sideband modulator means respectively responsive tosaid first and to said second difference signals for modulating arespective carrier signal in accordance with the amplitude of said firstand second difference signals, respectively, said carrier signals eachhaving a frequency such as to cause the upper sidebands of saidamplitude modulated signals to be outside the frequency range of saidfirst and second sum signals, said first and second single sidebandmodulator means each including output single sideband modulator means,envelope processor means responsive to the applied difference signal forproducing a modulating signal for the output single sideband modulator,and means for transmitting as an output signal the upper sideband of theamplitude modulated carrier signal produced by the output singlesideband modulator means, said envelope processor means comprisingsecond amplitude moDulator means for modulating a carrier signal inaccordance with the amplitude of the applied difference signal, filtermeans for transmitting the upper sideband of the amplitude modulatedcarrier signal from said second amplitude modulator means, means fordetecting the envelope of the transmitted upper sideband with carriersignal for producing a distorted replica of the difference signal, andmeans for applying said distorted replica of the difference signal tothe output single sideband modulator as a modulating signal, first andsecond equalizing circuit means for attenuating the low frequencycomponents of the first and second difference signals prior toapplication to their respective second amplitude modulator means andoperative to compensate for the presence of lower sideband signalinformation in the output of said filter means thereby to makesubstantially equal the modulation percentage in the output singlesideband modulator means for all frequency components, first means forcombining the first sum signal with the output signal of the firstsingle sideband modulator means, second means for combining the secondsum signal with the output signal of the second single sidebandmodulator means, and means for simultaneously recording the outputsignal of said first combining means on one wall of the single groove ofthe record disc and the output signal of said second combining means onthe other wall of the groove.